1. Field of the Invention
This invention relates to a maleic acid-modified polyolefin and a process for the preparation of the same.
2. Description of the Prior Arts
A polyolefin shows various excellent properties such as high heat stability, good chemical resistance and good weather resistance as a thermoplastic resin, and is used widely as a material of various articles such as blow molded products, injection-molded products, films and fibers. Particularly, as for a linear low-density polyethylene and polypropylene, a variety of usage thereof have been proposed because of their excellent physical properties.
The polyolefin finds a variety of usage in various arts as described above. Nevertheless, since the polyolefin has no polar functional group within its molecular structure, it shows extremely poor bonding property and printability, whereby the employment thereof is limited to a certain extent.
To improve the above-mentioned drawbacks, a method of modifying polyolefin with a polar compound such as maleic anhydride through graft-polymerization is generally utilized. In performing the graft-polymerization of maleic anhydride to polyolefin, there have been utilized various methods for initiating the graft-polymerization. For instance, there can be mentioned a method of irradiating a polyolefin with .gamma. rays, X-rays or high-speed cathode rays and a method of using a graft-polymerization initiator such as peroxide. The method of using peroxide is advantageous because no special equipment or device is required for initiating the graft-polymerization reaction. Accordingly, this method is widely used. Examples of the peroxide generally employable include benzoyl peroxide and tert-butyl peroxybenzoate.
In the modification of a polyolefin, it is desired that only the graft-polymeization reaction proceeds so as to graft-polymerize a modifying agent such as maleic acid to polyolefin, with keeping the excellent physical properties of polyolefin.
However, when a peroxide is employed as the reaction initiator, the peroxide serves as a graft polymerization initiator, but at the same time a portion thereof induces crosslinking reaction and decomposition reaction of polyolefin. Because of the crosslinking reaction or decomposition reaction, the inherent physical properties of polyolefin are deteriorated and the resulting modified product is unable to maintain the excellent physical properties of polyolefin. For example, the melt index of the modified product extraordinarily varies and the molecular-weight distribution broadens, so that the adhesiveness and moldability of the resulting modified product become unsatisfactory. As a result, the products produced using such unsatisfactory modified polyolefin are less appreciated on the market.
Further, the present inventors have found that when a linear low-density polyethylene is modified with maleic acid using peroxide as a catalyst, mechanical strength at a low temperature, which is one of the excellent properties of the linear low-density polyethylene, is hardly kept in the obtained modified polyethylene. In adition, peroxide decomposes as the reaction proceeds, and the decomposition product (e.g., butanol or other decomposition product) stains the modified product. For example, the modified product yields odor originating from the decomposition product, or turns to yellow because of the action of the decomposition product (or peroxide itself).
For the purpose of solving the above-mentioned problems caused by the crosslinking reaction or decomposition reaction which is induced by the use of peroxide, a process for the modification of polyolefin with maleic acid in which specific peroxide is employed under the specific conditions is proposed as stated, for example, in Japanese Patent Provisional Publication No. 51(1976)-31791. In this process, the above-mentioned problems are eliminated to a certain extent, but the crosslinking reaction and decomposition reaction of polyolefin are still induced, and thereby the excellent physical properties of polyolefin still decreases. Further, since the mechanism of reaction initiation of peroxide depends on the decomposition of the peroxide, the resulting modified product still not only yields odor caused by the decomposition product but also turns to yellow owing to the action of the decomposition product or peroxide itself.
Furthermore, according to the studies of the present inventors, it is considerably difficult to select the conditions for graft-polymerization of maleic acid to the polyolefin to an extent to approx. 100% by weight, when peroxide is used for the graft-polymerization reaction, and therefore the graft-polymerization reaction is hardly carried out with high efficiency as far as the conventional process is employed in the reaction. In addition, almost all kinds of peroxide including the aforementioned peroxide are very troublesome in handling thereof.